Method and apparatus for producing molten iron

Abstract

A system for producing molten iron from iron ores, including a direct reduction shaft furnace, a melting zone in a melting furnace having a coke bed in its lower portion and a charging and preheating chamber in its upper portion, and at least one reducing gas generation zone in communication with said melting zone.

Description

RELATED APPLICATIONS/PATENTS & INCORPORATION BY REFERENCE

A claim of priority is made to U.S. Provisional Application No. 60/507,160, filed Sep. 30, 2003.

Each of the applications and patents cited in this text, as well as each document or reference cited in each of the applications and patents (including during the prosecution of each issued patent; “application cited documents”), and each of the PCT and foreign applications or patents corresponding to and/or claiming priority from any of these applications and patents, and each of the documents cited or referenced in each of the application cited documents, are hereby expressly incorporated herein by reference. More generally, documents or references are cited in this text, either in a Reference List before the claims, or in the text itself; and, each of these documents or references (“herein-cited references”), as well as each document or reference cited in each of the herein-cited references (including any manufacturer's specifications, instructions, etc.), is hereby expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to the field of producing iron material usable for steel-making and other processes. Specifically, the present invention is directed to the combination of direct reduction of iron-oxides in a moving bed reduction reactor, a melting furnace where the reduced iron material is converted into molten iron, and a reduction gas generator furnace.

2. Description of the Related Art

It has long been recognized that developing a process capable of producing molten iron from iron ores at a competitive cost, as compared to the most efficient blast furnaces, is desirable. Blast furnaces continue to be the preferred method of producing molten iron in the steel industry due to their high productivity. Blast furnaces, however, require costly coke. This has several drawbacks, not the least of which is that coking coal is not found throughout the world. Thus, in many instances it must be imported at great cost to the iron producer. Further, the transformation of coal to coke requires a coking plant. Along with the cost associated with transforming coal into coke, coking plants are not considered environmentally friendly under new pollution regulations in many countries. As a result, many producers are looking for alternative methods of producing iron that will essentially eliminate the need for a separate coking plant, and a process for producing iron that requires much less coke.

A variety of techniques have been conceived for the production of molten iron that can be charged and refined in to steel in metallurgical furnaces. Two such examples are oxygen converters and electric arc furnaces. These processes aim to optimize traditional blast-furnace-type installation by substituting to some extent lower cost fuels for the traditional and more costly coke. But to date, these processes have failed to provide an operational plant capable of producing molten iron at competitive prices when compared with blast furnaces or with combined direct reduction and electric furnace facilities.

Examples of such proposals are U.S. Pat. No. 4,001,008 to Stift and U.S. Pat. No. 3,236,628 to L. Von Bogdandy. Stift discloses a method and apparatus for reducing iron ores in a shaft furnace, which is integrated to a melting furnace. Iron ores are introduced into the upper portion of the shaft without addition of coke and descend through the shaft in countercurrent to ascending reducing gases produced outside the furnace through partial combustion.

Hot reducing gases and powdered carbon are introduced toward a molten iron bath in the melting portion of the furnace, Stift's process however requires additional heat to be provided to the hot gases besides the heat produced by the partial combustion, and the integration of the reduction shaft with the melting furnace does not allow for providing the optimal conditions for the three main processes involved, namely: reduction of ores, melting of reduced ores and production of reducing gases. The Von Bogdandy process is similar in some respects also requiring additional heat for melting the reduced ores by providing electrical energy to the iron bath.

U.S. Patents Nos. 4,504,043 and 4,564,389 to Yamaoka et al. describe a process and apparatus for coal gasification and making pig iron. These patents disclose a melting furnace separate from the shaft reduction furnace, where the reducing gases are formed under conditions determined by the combustion of a fuel in the melting furnace. The melting furnace comprises a bed of coke filling most of the volume of the furnace. On top of the bed of coke a layer of reduced iron ore is laid which is melted by the heat of hot gases ascending through said bed of coke. High temperature reducing gases are produced by partial combustion of a fuel, e.g. pulverized coal, heavy oil, natural gas, etc. with oxygen in a plurality of burners which extend through the walls of the melting furnace. The molten iron and slag are discharged from the lower portion of the melting furnace and the hot gases are withdrawn and utilized for reducing iron ores in a reduction shaft associated with the melting furnace, or for other purposes.

The process of Yamaoka has not been utilized in the steel industry because it presents several disadvantages. For example, since the oxidation reactions of the fuel take place inside the melting furnace, some of the reactants may react with the bed of coke and consume the coke. Coke consumption causes an increase in operation costs both because of cost and because replenishment of the consumed coke requires special openings or operations in the use of the melting furnace.

U.S. Pat. No. 5,149,363 to Contrucci et al., discloses a process and a furnace for melting metal and smelting iron ores producing a molten iron material. This process has a shaft above a melting furnace. The utilization of the energy of gases produced by combustion of pulverized coal, liquid or gaseous hydrocarbons with oxygen in the melting furnace is improved by supplying air or oxygen at different locations in the shaft whereby heat is produced by oxidation of such gases which is used for promoting iron reduction of carbon-containing self-reducing pellets. When this process is used for melting metallized material, for example, scrap or pig iron, as in cupola furnaces, the coke consumption is significantly decreased because no coke is added to the burden, which thereby avoids any reaction of coke with the ascending reduction gases. However, this process does not separate the gas generation zone and equipment from the melting furnace.

Another known melting-gasifier furnace effects partial combustion of coal in a chamber coaxial to the melting furnace, whereby the gases produced impinge on the molten iron bath and then ascend counter-currently to iron containing particles descending through said furnace. This patent presents a separate chamber for partial combustion but presents a significant disadvantage because said chamber is located inside the melting furnace with considerable cost for special materials and design so that the structure withstands the high-temperature environment within said furnace.

Yet another process, which has been in operation for years, separates the reduction shaft furnace from the melting furnace but presents the disadvantage of forming a reaction chamber above the molten iron bath for combusting coal and producing heat and reducing gases which are utilized in the reduction furnace. Although this process is successful in not using coke, it however presents a great disadvantage of producing an excess of reducing gases that must be used for some other purposes, for example, electricity generation or for heating purposes. This process considered alone is not competitive and therefore its use has not spread as originally expected.

Accordingly, the present invention is directed to overcoming these and other shortcomings of the prior art.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to provide an improved system for producing molten iron usefull for steel making and other metallurgical processes.

It is another object of the invention to provide an improved system for producing molten iron with optimized operational conditions in three physically separated reaction zones or equipment, e.g. a reducing gas generation zone, a moving bed reduction zone and melting zone.

It is a further object of the invention to provide an improved system for producing molten iron adapted for its installation in conjunction with existing direct reduction plants, in order to substitute and efficiently utilize fuels having a lower cost that natural gas and therewith produce high quality pig-iron.

It is still another object of the invention to provide an improved system for producing molten iron adapted for its installation in electric furnace steel-making facilities, in order to charge molten metal to said electric furnaces and reduce electricity consumption as well as increasing their productivity.

Other objects of the invention will be evident to the skilled in the art or will be pointed out in the specification below.

SUMMARY OF THE INVENTION

The objects of the invention will be generally achieved by providing a system for producing molten iron from iron ores in form of lumps or pellets, comprising three separated reaction zones or furnaces: (1) a direct reduction shaft furnace, (2) a melting furnace, and (3) a reducing gas generation furnace; said system being characterized by: (a) producing a reduced intermediate product with a predetermined degree of metallization and carburization in said reduction furnace by reaction of said ores with a high-temperature reducing and carburizing gas; (b) charging said reduced intermediate product into said melting furnace upon a coke bed, whereby said reduced intermediate product is melted by contact with said high-temperature reducing gas; and (c) producing a reducing gas in a reducing gas generation zone by partial combustion of a hydrocarbon fuel with an oxygen-containing gas and steam; and (d) transferring said reducing gas into said melting furnace preventing any free oxygen from contacting said coke bed in said melting furnace, whereby any combustion of said coke bed is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

In this specification and the accompanying drawings, some preferred embodiments of the invention are shown and described, and various alternatives and modifications thereof have been suggested. It is to be understood that these are not intended to be exhaustive and that many other changes and modifications can be made within the scope of the invention.

The suggestions herein are selected and included for purposes of illustration in order that others skilled in the art will more fully understand the invention and the principles thereof and will thus be enabled to modify it in a variety of forms, each as may be best suited to the conditions of a particular use.

In the following detailed description, reference will be made to the accompanying drawings, wherein:

FIG. 1 shows a schematic diagram of one embodiment of the present invention for producing the reducing gases and for distributing said gases in the melting furnace.

FIG. 2 shows a schematic diagram of a second embodiment of the present invention for producing the reducing gases and for distributing said gases in the melting furnace.

FIG. 3 is a graph showing the preferred ratio of oxygen to natural gas for melting iron according to one embodiment of the present invention.

FIG. 4 is a schematic diagram of a further embodiment of the present invention for producing the reduction gases and for distributing said gases in the melting furnace where DRI is produced remotely.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will now be described with reference to the Figures. Referring to FIG. 1, a melting furnace 10 having a crucible section 12 and a charging shaft 14 is shown. A bed of coke 16 fills a major portion of the crucible section 12 providing a porous support for a bed 18 of metallic-iron-containing particles which descend through the shaft section 14 as said iron particles melt down. The high-temperature reducing gas 20 produced in a plurality of gas generating zones 22 passes through the coke bed 16 with a composition comprising in its major part hydrogen and carbon monoxide, however, the mixture may contain some carbon dioxide and water, the classic by-products of combustion, and also some methane or other un-burnt fuel. The actual composition of the reducing gases 20 will depend on the type of fuel utilized and will be adjusted to a desired reducing potential value by reacting a fuel 24 and a free oxygen-containing gas 26. Flow rates of fuel and oxygen-gas are regulated by suitable control valves 33 and 35.

Fuel 24 may be any hydrocarbon in solid, liquid or gaseous form. Natural gas may be utilized with advantages except for its unavailability in many places and its wide price fluctuation. Liquid hydrocarbons, such as naphtha, “bunker C” oil, and waste oils can be utilized, as well as coal ground to a suitable particle size so as to be handled as slurry with a liquid hydrocarbon and fed to burners 30.

Pure oxygen is preferred as oxidant for partially combusting the fuel, since the presence of nitrogen in the reducing gases will limit the amount of reducing gas, which may be recycled to the reduction reactor, but air and oxygen-enriched air in burners 44 can also be used for melting the reduced materials.

The reacting materials in burner 30 produce a high-temperature flame 20, between about 2,000° C. and about 2,500° C. The gas-generating zones 22 are designed so that, preferably, the flame does not directly impinge in its refractory-lined walls. The gas-generating zones 22 are also designed so that the reaction volume allows for total consumption of the free-oxygen-containing gas before such gases are introduced into the melting furnace and contact the coke bed 16. To this end, said gas-generating zone has a length of at least about 0.6 m., preferably more than about 0.8 m. However, other lengths of the gas-generating zone will be apparent to those of skill in the art and are contemplated to be within the scope of the instant invention. The exact length is not by itself as important to the process of the present invention as combustion to a point of using all free oxygen and preventing contact of free-oxygen with the coke.

The goal of this length of the gas-generating zone 22 is to minimize the consumption of the coke and consequently the amount of coke, which must be replaced in the melting furnace, is minimized. This goal is largely achieved by ensuring that substantially all of the free oxygen is used in combustion prior to the reducing gases contacting the coke. By accomplishing this, there is nearly no oxygen remaining to oxidize or burn the coke. By preventing the coke from burning, a major cost from a potentially consumable material of the iron making process is minimized, thereby reducing the overall cost of the process. Thus through the use of other fuels, such as coal, oil, petcoke and the like, which are much less expensive, the efficiency and cost of iron making is substantially reduced.

The reduced-iron-containing material (DRI) 18 is melted and flows down through the coke bed 16 to the bottom portion of melting furnace 10 wherefrom it is extracted through discharge channel 32 into molds (not shown) in a manner known in the art.

The melting furnace 10 is preferably operated under pressure, in the order of 1 to 10 kg/cm², preferably from about 4 to 6 kg/cm², so that the effluent gas 34 from outlet 36 may be injected into the reducing gas circuit of reduction reactor 38, without need of any pumping means. The effluent gas is at a temperature of about 400 to 800° C.

Iron-containing materials: DRI (sponge iron) 39, and other materials, for example, limestone 48, limestone and fluxes 50, coke 52 and DRI or scrap iron 53 are charged into melting furnace 10 through a bins-and-valve system comprising an atmospheric bin 40, a pressurized bin 42 and seal valves 44 and 46. Materials are charged to bin 40 and transferred to bin 42 by opening valve 44 while valve 46 is closed and then closing valve 44, pressurizing bin 42 and opening valve 46, in a manner known in the industry. Coke is charged in the amount necessary to make-up coke bed 16 as some coke is incidentally gasified by the heat of gases 20. Iron briquettes, iron scrap and the like may also be charged to melting furnace 10 for melting.

Reduction reactor 38 comprises an upper reduction zone 54 and a lower discharge zone 56. Iron oxides in the form of lumps, pellets or mixtures thereof in suitable sizes, are charged into reduction zone 54 by any suitable means known in the art depending on the level of pressure under which the reactor is operated, for example a bins and valves means or a charging leg filled with an inert sealing gas. Iron oxides pellets descend by gravity through reduction zone 54 in contact with an ascending stream of reducing gases at a temperature between about900° C. and about 1100° C., whereby iron oxides are converted to metallic iron. After reduction, the DRI or sponge iron is charged into the melting furnace 10 as needed.

It will be evident by those skilled in the art that a different type of reduction reactor, other than the moving bed reactor 38, may also be utilized according to the invention, for example a fixed bed or a fluidized bed reactor.

After reduction of the iron pellets, exhausted reducing gas 60 is withdrawn from reactor 38 and passes through heat exchanger 62 where water from source 64 is made into steam 65 that can be used in burners 24 or CO₂ removal system 78. The exhausted reducing gas 60 is then cooled down in cooler 66 by, for example, contacting said gas with water. This process condenses any residual water content out of the exhausted reducing gas. A minor portion 68 of the cooled gas, regulated by control valve 70, is purged from the reduction circuit thus eliminating inert gases and simultaneously regulating the pressure of the system.

The major portion 72 of the cooled gas is then pumped by compressor means 74 and is fed to a CO₂ removal system 78 in order to remove carbon dioxide. This has the effect of regenerating the reducing potential of the gas. The CO2 removal system 78 may be of the absorption type where the gas is contacted with a liquid or of the adsorption type (pressure or volume swing adsorption). Recycled reducing gas 80, if necessary, is then combined with natural gas 82 from a suitable source 84 and heated in gas heater 90 to a temperature in the range from about 750° C. to about 950° C. Natural gas is also utilized, as required, in the burners of heater 90, combined with a portion 88 of reducing gas produced in melting furnace 10. Hot gas from heater 90 is combined in variable proportions with more natural gas 92 from a suitable source 94, a free-oxygen-containing gas 96 from source 98, which may be molecular oxygen or oxygen-enriched air, and effluent gasses 34 from the melting furnace 10. Free-oxygen injection produces a partial combustion of hydrocarbons in the hot gas 58 increasing its temperature in the range from about 1000° C. to about 1150° C. and is then fed to the reduction reactor 38 for producing the DRI or sponge iron 39.

FIG. 2 depicts a further embodiment of the present invention, wherein like numbers designate similar or equivalent elements described with reference to FIG. 1. The apparatus depicted in FIG. 2 is used to produce molten iron or pig iron from iron ore. The apparatus includes a melting furnace 10, a reduction or shaft furnace 38, and a reduction gas-generating furnace 21.

The melting furnace 10 includes a coke bed 16 on which reduced iron 18 or DRI or sponge iron is deposited for melting. Iron is conveyed into the reduction furnace 38 by conventional means, and is therein reduced to form DRI. Heated gases are provided to the reduction chamber 38 to heat the iron ore, in part through the use of a reducing gas generator 21. The reducing gas generator 21 includes one or more burners 31 which burn hydrocarbon fuels including but not limited to coal, oil, petcoke, and the like. The burner 31 is connected to a variety of piping systems 24, 26 that can include oxygen systems, fuel systems, steam systems, and others. Oxygen is used to effect a partial combustion of the hydrocarbons which are supplied by the fuel systems and steam may also be used to both atomize the fuel for combustion and to increase the temperature of the mixture before combustion. The partial burning of a fuel such as petcoke creates a high temperature, high-pressure reduction gas, which is injected into the melting furnace 10 through a passage 23. The gas is typically above about 14000 C and has a reducing composition before it contacts the coke bed 16. These gases are distributed around the periphery of the crucible section 12 by a gas distribution plenum 25 formed in the refractory lining 27 and having nozzles (not shown) around said melting zone 12. In one embodiment, valves 33 and 35 regulate the flow rates of fuel 24 and oxygen-gas 26 respectively, in order to adjust the temperature and composition of the reducing gas produced. Gas generating chamber 21 is provided with a bottom outlet 100 and a shut-off valve 102 for withdrawing ashes, slag and impurities, which may accumulate therein.

The hot gases rise through coke bed 16 melting the DRI deposited thereon in counter-current flow. The melted iron collects at the bottom of the melting furnace 10 and exits through connection 32 to molds or for use in further processing such as steel making.

After passing through the DRI that has been charged into the melting furnace 10, the reduction gas is vented through a connection pipe 36 to the reduction furnace 38. In the reduction furnace the reduction gas is used to transform the iron ore into DRI for use in the melting furnace. The reduction furnace 38 operates substantially the same as the reduction furnace described with relation to the embodiment shown in FIG. 1.

Referring to FIG. 3, the graph shows the preferred operating range of the ratio of volume of oxygen to volume of natural gas in the gas generating zones 22 or the gas generating furnace 21 so that the temperature of the gases reach such a level as to melt the DRI, and also that the amount of heat produced is sufficient to melt the required amount of DRI and at the same time provide sufficient amounts of reducing gases (H2+CO) for reducing the iron oxides in furnace 38 to DRI. It is known that the temperature and amount of heat will increase with an increase in the ratio of oxygen to natural gas, but this increase of oxidation will decrease the reducing quality of the reduction gas and therefore the amount of reducing agents available for reduction. Applicants have found that the preferred range of the ratio of oxidant to fuel for operating a melting furnace must comply with the above-mentioned requirements for the process to be economically attractive. Although the concept of preferred range of oxygen to fuel ratio has been herein illustrated for natural gas it can be applied to other fuels. For natural gas the preferred range is from about 0.86 to about 1.04.

FIG. 4 depicts an embodiment of the present invention where the reduced-iron-containing material is produced at a location remote from the molten-iron producing plant and charged into the melting furnace via DRI bin 39. This embodiment may be useful in an instance where a steel making plant buys DRI from DRI-merchant plants. As shown in FIG. 4, the hot reducing gases produced in the melting furnace 10 may then be utilized as fuel, synthesis gas or power generation after having been used in the melting furnace 10 and vented by connection pipe 36. Such an embodiment may also be useful in increasing the productivity of electric arc furnaces where molten iron is charged into an electric furnace thereby reducing melting time and electricity consumption.

In the schematic diagram FIG. 4, of the present invention there is shown an application where the reducing gas effluent 34 from said melting furnace is recycled therein. Numerals in FIG. 4 designate the same elements as in FIG. 1. Reducing gases 34 effluent from melting furnace 10 are cooled and cleaned in heat exchanger 62 and cooler 66 and are then recycled through compressor means 74, passed through a CO₂ removal system 78 and then heated to a temperature above about 800° C. and then recycled to said melting furnace 10 in order to utilized them as much as possible for melting DRI. This gas recycle increases the efficiency of a melting furnace.

It has thus been shown that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, because certain changes may be made in carrying out the above method and in the construction(s) set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

1. A system for producing molten iron from iron ores, comprising:

(1) a direct reduction shaft furnace;

(2) a melting zone in a melting furnace having a coke bed in its lower portion and a charging and preheating chamber in its upper portion; and

(3) at least one reducing gas generation zone communicating with said melting zone, wherein said system: (a) produces a reduced intermediate product with a predetermined degree of metallization and carburization in said reduction furnace by reaction of said ores with a high-temperature reducing and carburizing gas; (b) charges said reduced intermediate product into said melting furnace upon said coke bed, whereby said reduced intermediate product is melted by contact with said high-temperature reducing gas; (c) produces a reducing gas in said reducing gas generation zone by partial combustion of a hydrocarbon fuel with an oxygen-containing gas; and (d) transfers said reducing gas into said melting furnace preventing any free oxygen from contacting said coke bed in said melting furnace.

2. The system for producing molten iron from iron ores according to claim 1, wherein said reducing gas is produced in a partial combustion chamber so that the hydrocarbon fuel, and oxygen-containing gas react an combust to substantially eliminate the presence of any free oxygen before contacting said coke bed.

3. The system for producing molten iron from iron ores according to claim 2, wherein said hydrocarbon fuel is natural gas.

4. The system for producing molten iron from iron ores according to claim 3, wherein the ratio of the volumetric flow rate of the oxygen-containing gas to the flow rate of said natural gas is in the range from about 0.86 to about 1.04.

5. The system for producing molten iron from iron ores according to claim 2, wherein said hydrocarbon fuel is liquid.

6. A system for producing molten iron from iron ores according to claim 2, wherein said hydrocarbon fuel is pet-coke.

7. A system for producing molten iron from iron ores according to claim 2, wherein said hydrocarbon fuel is coal.

8. The system for producing molten iron from iron ores according to claim 1, wherein said system:

(e) withdraws reacted reducing gas from said reduction furnace;

(f) regenerates the reducing potential of said withdrawn reducing gas by cooling and removing at least a portion of its water content;

(g) heats a portion of said regenerated withdrawn reducing gas to a temperature above 800° C.; and

(h) feeds said heated regenerated withdrawn reducing gas to the reduction furnace.

8. The system for producing molten iron from iron ores according to claim 1, wherein said reducing gas is produced in a single partial combustion chamber and fed to said PATENT 120194-4420.1 melting furnace through a plurality of inlet ports formed and located in the refractory lining of said melting furnace.

9. The system for producing molten iron from iron ores according to claim 1, wherein said reducing gas being is produced in a plurality of partial combustion chambers connecting to said melting furnace through a plurality of inlet ports formed through the wall of said melting furnace.

10. The system for producing molten iron from ores according to claim 1, wherein said inlet ports are located in said melting furnace so that the high-temperature reducing gas is fed to contact said coke and ascend through a portion of said coke bed and through said preheating chamber of said melting furnace.

11. The system for producing molten iron according to claim 10, wherein said iron-containing material is DRI produced in a location remote from said melting furnace.

12. The system for producing molten iron according to claim 11, wherein said reducing gas effluent from said melting furnace is further used as fuel, power generation or synthesis gas in other chemical processes.

13. The system for producing molten iron according to claims 8, wherein the length of said partial combustion chamber is at least about 0.6 m.

14. The system for producing molten iron according to claims 8, wherein the length of said partial combustion chamber is at least about 0.8 m.

15. The system for producing molten iron according to claim 1, wherein said melting furnace is located at a remote distance from said direct reduction shaft furnace, wherein said system:

(a) withdraws effluent gases from said melting furnace;

(b) cools and cleans said effluent gases;

(c) separates CO2 from said cooled effluent gases;

(d) heats said cooled effluent gases to a temperature above about 600° C.; and

(e) recycles said heated reducing gases to said melting furnace.

16. The system for producing molten iron according to claim 15, wherein a portion of said cooled and cleaned effluent gases from said melting furnace are utilized as fuel for heating said recycled gases.